The invention relates to load sensors for AC induction motors.
The invention arose during development efforts to provide a load sensor in various applications. One such application is a gas furnace where it is desired that there be sufficient airflow before a gas jet is turned on. This is accomplished by sensing blower or fan motor load. Multiple ranges of motor load are sensed. If the motor load is too high, a load-too-high signal is generated, indicating insufficient airflow, and the gas jet is inhibited from being turned on. If the motor load is within a given acceptable range, a load-OK signal is generated, indicating sufficient airflow, and the gas jet is enabled. If the motor load is too low, for example if the fan becomes loose on the motor shaft, a load-too-low signal is generated, indicating insufficient airflow, and the gas jet is disabled. In the preferred embodiment, the sensing of multiple ranges of blower or fan motor load is accomplished by sensing the speed of the motor driving the blower or fan, which in turn is accomplished by sensing the magnitude of auxiliary winding voltage of the motor.
It is known in the prior art to sense motor load as a function of motor speed which in turn is a function of auxiliary winding voltage. For example, it is known in motor starting circuitry to sense increasing auxiliary winding voltage, and at a given cut-out threshold, actuate a disconnect switch to de-energize the auxiliary winding at cut-out speed. It is also known in such starting circuitry to re-energize the auxiliary winding at cut-in speed to accelerate or restart the motor from a stall or overload condition. Examples of such starting circuitry are shown in U.S. Pat. Nos. 4,622,506, 4,658,195, 4,670,697, 4,719,399, 4,751,449, 4,751,450, 4,782,278, 5,017,853, assigned to the assignee of the present invention.
The present invention applies various aspects of the above noted motor starting technology and improvements thereof to multi-level load sensing applications. The present invention may be used alone or in combination with motor starting circuitry. In the preferred embodiment, the invention is used in a permanent split capacitor motor, though the invention is usable in other motors including a start run capacitor motor, a start capacitor motor, and a split phase motor.